Grundig TK20 and Truvox R82. Help Required Please.
 

Hi Everyone,
I have a Grundig TK20 and a Truvox R82 reel to reel tape recorder. Both are fine mechanically but both have audio issues. The Grundig plays with good volume for around 20 minutes before slowly fading until it reaches a point that increasing the volume control has no effect. The Truvox had a similar problem but has now changed to a hum which varies with the volume control and the audio level is extremely low, hardly audible in fact.
If anyone can pinpoint what is likely needed to be replaced and it is a simple job I'll give it a go, otherwise is there anyone out there in my area, Merseyside, who would be prepared to take a look at them for me. I have recordings dating back to the early 1960's when I was in a band and I would like to be able to listen to them again!

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Two points here - the most likely culprit on either machine is likely to be ageing capacitors, but the best remedy is not wholesale replacement without further investigation. If your main purpose is to preserve old tapes, it may be more useful to have them transferred to digital files. I do this for a living, as do others...

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Hi Ted, I have the facility to digitise the tapes if I can get the machines to work. I would like the Truvox in working order preferably as it is two speed and most of my tapes are 71/2 ips recordings. Thanks for your suggestions anyway.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Hello Terry,
As Ted suggests it could well be bad capacitors but not easy to prove without various tests, which would involve working on open chassis with various potential dangerous voltages present, so not advisable unless you know what you are doing, for safety reasons.

Some people would recommend wholesale capacitor replacement but a big job unless you really know what you are doing and there is potential for creating new problems and some maybe even many could be fine anyway.

There are 2 main areas/capacitor types to check out:-

1. The rectified DC + HT voltage, should be around +285V dc with a several volts max AC mains ripple. Should be checked/monitored for the period that the volume issue develops, for the R82 this is at electrolytic capacitor C37 (there are also lower associated voltages after this). For the TK 20 the HT voltage should be around +260 volts at C31. Low HT voltage/high ripple could indicate bad reservoir/smoothing electrolytic capacitors.

2. Grid coupling capacitors, normally paper (di-electric type) in these vintage of machines. There is a sticky Thread about these in the "Components and circuits" category. Basically they should not pass DC voltage, so little if any +ve voltage should be measured at the Control Grids of the valves, particularly important at the output valve.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Hello David,
Thanks for the info, and NO I certainly won't be messing with open chassis measuring voltages, I don't have the expertise for that. You would probably find the fault in no time but unfortuately you're too far away for me to let you have a look, but, I can replace components easily enough. On the Truvox is C37 likely to be a can type or just a standard axial electrolytic?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

If still original part then most likely will be a metal can capacitor. I see Terry that you posted the schematic in 2013.

Looking at the schematic you will see that C37 is 32uF, this is the filter capacitor after the reservoir capacitor C38, also 32uF, so quite possibly a dual can capacitor. The first smoothing capacitor after C37 is C33 also 32uF, so it is possible that it is a triple 32uF can capacitor.

The Service manual only has an abbreviated spares listing and these capacitors are not listed, so you will need to check out exactly what capacitor (s) are fitted.

These type of capacitors can hold a significant charge after being powered off, so always allow at least 15 minutes (and disconnect the mains cable from the wall outlet) after power down to allow them to discharge before doing any replacement work.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Just noticed that there is another 32uF (C17) at the start, so probably the actual reservoir cap, very rare but now there is the possibility of the can capacitor being a quad 32uF.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Funnily enough David, I downloaded the manual and found the same as you with the parts list, so I did a list of all the caps shown on the schematic and then did a visual inspection to identify which was which and to get the working voltages. I couldn't locate any of the 4 32uf caps listed on the schematic. However, there are 2 Large upright cans which are presumably what you are referring to. I'll check if there are any markings on them and get back to you.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

C17 is directly off the +ve output of the bridge rectifier EC25, then following the connection up to R49 (330 ohm) the other side of R49 connects to C38, then onto R48 (330 Ohm) the output of which connects to C37, which connects to R44 (2.2k) the output of which connects to C33.

Possibly from your description there may be two 32uF dual cans.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Correct! I've just had a look and there are two cans 40+40uF rated at 350v. They are roughly 3 inches high by 1 inch diameter. I presume these are going to be hard to come by which means replacing them with made up DIY substitutes. From tracing the wiring C37 and C38 are in one and C17 and C33 in the other.

See pics here. https://www.vintage-radio.net/forum/...pictureid=4506
https://www.vintage-radio.net/forum/...pictureid=4507

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

You can still get new parts from a few places, such as -

https://www.hotroxuk.com/jj-capacitors-caps.html

The fact that they are 40uF I wonder if that could mean that they have been replaced before (if so could well be good), do they look newish/modern, has the soldering been disturbed ?

Cannot view the pics in your links.

Update - after more attempts now able to view your pictures, difficult to be sure but they do not look new/modern.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

No, they're all original. It's never been faulty before and I've had it from new!
Could this be a suitable replacement?
https://www.tubeampdoctor.com/en/32-...radial-35x50mm

Sorry, you beat me too it with HotRox! I've had valves from them in the past for my Vox amp.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Those 2 large rectangular waxy looking capacitors on the RHS of the first picture could be candidates for replacement, are you able to identify them ?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Good that you have had it from new, as you know all its previous history.

Yes your linked one look fine as well. What ever you get, make sure they will fit in your existing clamps and not too long so as not to foul anything.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

The larger of the 2 waxy caps is C28 and the other C21 associated with V3 the ECC82. The new ones are 35mm diameter whereas the existing ones are 25mm, height is ok.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

OK so low (relatively) value ( I assume 1,000 on the schematic = 1,000pF ?) used on the Bias Oscillator, in that case I would not worry about them at this stage.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Yes they are 1000pF. I notice TubeAmp have clamp to go with the capacitor. Presumably I could use this even it meant drilling new holes for the screws and as long as the terminals are clear of the chassis.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Yes if there is space for the larger clamps and mounting holes.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

I'll order 2 together with clamps and start with them and see what difference they make. Hopefully it will sort the problem. I really appreciate you help with this as I may be able to wield a hot soldering rod but I'm certainly no hot electronics whizz! But I am learning with the help of the likes of yourself. Many thanks!
Pic of the Truvox in its case. https://www.vintage-radio.net/forum/...pictureid=4508

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Looks a nice machine.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

While you are in there I would recommend you visually check out capacitor C35 (0.01uF) to see if it is a paper type, the grid coupling capacitor for the output pentode V5B (ECL86). Worst case if the capacitor is very electrically leaky then could cause circuit damage.

For good measure also would check out C12 (0.1uF) the grid coupling capacitor for V5A the triode section of the V5 valve.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Hi David,
I read the section about grid coupling capacitors or "that Cap" as it seems to be referred to. You mentioned about these earlier. Would that be C35 on the ECL86, V5A?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Oh, your post popped up just as I posted my question! Thanks.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Had a visual check on C35 and C12. C35 looks to be covered in a waxy substance and doesn't look brilliant but that means nothing I suppose. C12 looks to be in better condition. Both are in really awkward locations being underneath other components. Can you tell if they are paper from the pictures?
Here is C35.https://www.vintage-radio.net/forum/...pictureid=4514

Here is C12.https://www.vintage-radio.net/forum/...pictureid=4515

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

C35 almost certainly looks wax covered paper so I would replace. You can cut through the legs to make it easier to remove the old capacitor and unsolder the leg remnants at the valve pins.

C12 looks like a yellow mustard type which is not paper (polyester foil/film type), these normally do not leak and should be fine.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Thanks David. I'm just waiting for the parts to arrive now so I can make a start! I assume it would be best to start with the two cans and see what difference they make, and then progress on from there, right? Which type (construction) of cap would it best to replace C35 with?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Almost any modern axial Polyester/Ceramic/Polypropylene metal film etc.

I personally like Polypropylene.

If possible give C35 some priority because if it was really bad from electrical leakage point of view it could damage the output valve/output transformer (worst case).

When replacing the can capacitors would make sense to measure (if you have a multimeter) the resistance of the associated resistors connected to them, just to make sure none are really abnormally low/high.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Good morning David,
Thanks for the recommendations. I've checked the resistors associated with both cans and they measure ok, so while I was at it I checked all resistors and the only one that reads incorrect is R41 2.2M which only reads 0.05 on the 20M scale. It's colour coded red, red, green in the photo of C35, second from right. Am I not measuring correctly or is it a faulty one?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Just double checked with a better multimeter and the result is the same. Checked R1 which is also 2.2M and that reads ok, so I guess R41 has gone open circuit has it?

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Maybe a poor connection to R41 and R41 is open circuit (in effect)

Lawrence.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Depends if your 0.05 reading is a 0.05 ohms, i.e. effectively a short circuit or 0.05 MOhms (50k) or effectively an open circuit indication on that particular meter.

Terry you need to disconnect one end of R41 and recheck. If the volume control R40 (think it is volume) is at one end and switch S2 is made (do not know what this is) there is a direct path from the top end of R41 to earth/ground so then giving a very low resistance reading across R41 (bottom end of R41 connects to ground).

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

He's deleted the 51meg measurement he quoted from his post....

Lawrence.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

So far as I can make out R40 is the source monitor volume control in record and a tone control in playback, S2 is shown in record mode.

Lawrence.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

I did not see the 51Mohm stated.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

He stated that both his test meters indicated 51meg when connected across R41, after which the post was edited for whatever reason.

Lawrence.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Yes as you say in that case maybe a bad connection or R41 open circuit. The repeat measurement of R41 with one end disconnected should prove one way or the other.

Now see the reference in the manual to the tone control/source monitor volume.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

I don't have the full manual but do you know why pin 1 of the output DIN socket is DC connected to the grid of V5a?

Lawrence.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Hi David,
R40 is a tone/monitor control and the switch is the rec/play mode switch. I've put the switch in record mode and the reading is zero which would indicate a direct route to ground, so in play mode with the switch off I get a reading of 0.05M which varies if I turn the tone control R40 to a max of 0.42M. So without as you say disconnecting it I'm not going to get a true reading, plus I'm wondering if this might be a bit of a red herring anyway!

UPDATE: It was a red herring. Cut it carefully on the earth connection side so I could re-solder it together and got a reading of 2.6M, so it's ok.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

I wondered exactly the same when I saw it, do not know, maybe Terry has the User manual which may explain.

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Yes looks like a red herring.

The parallel calculation of 2.2M with 547k (R40 500k + R39 47k) = 440k (0.44M).

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

David, see my update in my last posting. Just going to re-solder R41 together again and then it's a wait till I get all the bits and pieces together to start on the waxy cap and the two cans. (Or should that be 'toucans' as in toucans of Guinness please!

Don't know what pin 1 on the DIN socket is for or why there's a connection from the tone/monitor control. I know pins 2 & 5 are to connect a 15ohm loudspeaker as I used to have one connected to it, and how much better the sound was than the 3 ohm!

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

OK that is good, so R41 around 20% high and around 10% above its max tolerance of 10% (silver band) but very typical of these old carbon resistors, nothing to worry about.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Can't thank you enough David, and others who've contributed. All I have to do now, as I say, is wait for the parts to arrive.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Fingers and toes crossed the new parts help :)

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

I certainly hope so! I'll be back on when the jobs completed - succesfully hopefully! Thanks again.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Here's a couple of articles from the Tape Recorder Magazine which I kept from 1963 and 1965. They may be of use to anyone who has or may come into contact with Truvox tape recorders and they need a bit of TLC!

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

Interesting and useful articles.

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

UPDATE: Have installed the two replacements for the two 32+32uF cans. As you can see from the pictures I was very lucky with the hole spacing as the new clamping rings had enough adjustment to reuse the existing holes which saved me a bit of work! I used some washers to act as spacers to make sure the body of the new caps were not in contact with the chassis. I probably didn't need to as they are covered with a plastic cover, but it just made me feel better! I'm just waiting for the .01uF caps to arrive so I can change that one across pins 8 & 9 of the ECL86 output valve. I could try it now but I think I prefer to wait until that's done. Why do JJ Electronics mark their caps with an M instead of uF? I assume it's M for microfarad?
One thing that did surprise me was that at some time the ECC81 has been replaced with an ECC82! That can't be doing any good can it? Anyway, I've replaced it with a new ECC81.
https://www.vintage-radio.net/forum/...pictureid=4520
https://www.vintage-radio.net/forum/...pictureid=4521
https://www.vintage-radio.net/forum/...pictureid=4522

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

M and sometimes m was used on older documentation for uF (Micro Farad) and maybe also on some vintage capacitors, so I guess this is why JJ Electronics annotate their capacitors that way.

The ECC81 and ECC82 I am pretty sure are pin compatible and basically the same type valve with the ECC82 being a lower gain version. Should not cause a problem and unless in a critical circuit may well function just as well. Having said that I also would be happier with correct valve fitted.

I see the Bias Oscillator V3 is a ECC82, V2 (ECC81) has not been swapped over with V3 has it ?

David

Re: Grundig TK20 and Truvox R82. Help Required Please.
 

No, I checked that and it was an ECC82 which is what alerted me to the fact that the other one should have been an ECC81. If that ECC82 is more or less compatible then it'll do as a standby. I see somewhere on here that the ECL86 is regarded as somewhat prone to failure! I hope mine is ok as they are not cheap at £36! Just relaxing now with a nice single malt.

Terry